The 78 and 78E are variable μ RF pentodes on the American UX6 base. The RCA Receiving Tube manual describes the 78 as being electrically identical to the later IO based 6K7 apart from the capacitances, thus this is one of the Magic Three valves.
The outer shield hides the inner electrodes.
Looking up from the base, the grids can be seen to be of a significantly smaller diameter than the screen.
The classic envelope is 38 mm in diameter, and excluding the UX6 base pins is 101 mm tall.
References: Datasheet, 1040, 40553002, & 1043. Type 78 was first introduced in 1933.